This download looks like a map. You need to place it in your maps folder inside the Garry's Mod folder.
You should end up with a directory structure similar to below
Don't know where the Garry's Mod folder is or how to extract zip files? See more useful installation tips.
frutela.aca, mgamerytlolxd, mgamerytlolxd, Joaco_BG, Kurstah, Sukisyo, sorenyip22, SpyHunter, Send_Me_Dem_Noots, Kinghuz, applepielord333, Asansörden Gelen Tehlike, WhipRBLX, SOVIET LOBSTER, brin900, Alvinic, JastPlaying, glimmerman, lwhiteside23, Private_Cauliflour, PatricPlays, PatricPlays, CubicTuxedo, Flames, senzamak, csrss